Remote trace gas detection and analysis

ABSTRACT

A system for the remote detection and analysis of trace chemical agents in the air. A beam of electromagnetic radiation is used to radiate a cloud. The radiation energy that is absorbed by the cloud is thermalized by collisional energy transfer between the molecules that absorb the radiation. Increases in the cloud temperature increase the emission intensity of the molecules against the background, resulting in improved detection of the target molecules. A tracking telescope is used to collect the thermal emissions generated by the radiation beam. A spectrometer is used to resolve the emissions from the cloud and generate an emissions spectrum. The wavelength of the electromagnetic radiation can be selected to be in resonance with the absorption lines of water or oxygen molecules in the cloud, or to be in resonance with absorption lines of known target molecules in the cloud to generate the heat.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to a method for detecting tracegases in the air and, more particularly, to a method of radiating achemical cloud to heat the cloud and increase its temperature relativeto the background, and then detecting chemicals in the cloud byspectroscopy.

[0003] 2. Discussion of the Related Art

[0004] It is known in the art to detect certain constituents in achemical cloud in the air by spectral analysis of the molecules makingup the cloud. This type of chemical detection has many applications,including detecting natural gas leaks from underground pipes, chemicalclouds from chemical spills, volatile organic vapor (VOC) from chemicalprocesses, pollution from smoke stacks and the like, military chemicalwarfare agents, and other toxic gases present in the air. Typically,this type of spectral analysis of a chemical cloud is performedremotely, sometimes up to 10-20 km away, because the constituents in thecloud may be toxic, and thus a threat to health, or it may not bepossible to directly detect the chemical cloud. The distance thedetecting instrument has to be from the cloud for this remote type ofpassive sensing depends on the particular application, and differentsystems exist for different applications.

[0005] To perform this type of detection and analysis, a spectrometer,such as a Fourier transform infrared (FTIR) spectrometer, is directedtowards the chemical agent cloud from a remote location, so that itpassively receives emissions therefrom. If the chemical cloud is warmerthan the background, such as sky, mountains, or other terrain, along thefield-of-view of the spectrometer, target molecules in the cloud willexhibit emissions having an energy greater than the background emissionsfrom the sky. If the chemical cloud is the same temperature as the sky,the target molecules within the cloud are absorbing photons at the samerate that they are emitting photons, so that there is no net energyexchange between the cloud and the background, and no differencerelative to the background. As the temperature of the cloud increases,more photons are released from the chemicals in the cloud, which areavailable to be received by the spectrometer.

[0006] The spectral display generated by the spectrometer from theemissions provides emission lines and bands at certain wavelengths thatis indicative of the atoms and molecules in the cloud. Because eachmaterial has its own spectral “fingerprint” representative of itsmolecules, the detected spectral display can be compared to a known“fingerprint” of a particular chemical to determine if that chemicalexists in the cloud.

[0007] A problem exists with the passive remote sensing techniques thatare currently used in the art because the temperature difference betweenthe chemical cloud and the sky is often very small. In many cases, thetemperature of the cloud is only about 2-3° C. warmer than thetemperature of the background. Because of such a small temperaturedifference, the detectable emissions from the cloud is typically veryweak. This results in a poor signal-to-noise ratio, and thus poordetection sensitivity and possibly a high false alarm rate.

[0008] What is needed is a remote chemical detection system that causesthe chemical cloud to be heated so that the temperature of the cloud issignificantly different than the background. It is therefore an objectof the present invention to provide a remote chemical detection systemof this type.

SUMMARY OF THE INVENTION

[0009] In accordance with the teachings of the present invention, atechnique for the remote detection and analysis of trace chemicals inthe air is disclosed. A beam of electromagnetic radiation from anelectromagnetic radiation source is used to radiate a suspected chemicalcloud. The radiation energy that is absorbed by the cloud is quicklythermalized due to a rapid collision energy transfer between themolecules that absorb the radiation and the surrounding air molecules.This collisional energy redistribution will result in heating thechemicals in the cloud. An increase in the temperature of the cloud willincrease the emission intensity of the molecules against the background,resulting in an improvement in the detection of the chemicals.

[0010] A tracking telescope is then used to collect the thermalemissions of the target molecules generated by the radiation. Thetracking telescope can be located in the vicinity of the electromagneticradiation source such that the viewing axis of the telescope ispreferably coaxial with the propagation direction of the electromagneticradiation. A spectrometer, such as an FTIR spectrometer, is used toresolve the emissions from the cloud that are enhanced by the radiationand to generate an emissions spectrum. The emissions spectrum is used toidentify suspect molecules in the cloud by comparing the detectedemissions to the known “fingerprint” vibrational spectrum of the suspectmolecules.

[0011] The electromagnetic radiation source, telescope and FTIRspectrometer can be housed on a platform to scan over a wide area forsurveillance of chemical clouds. Alternatively, the tracking telescopeand the FTIR spectrometer can be located at a separate location fromthat of the electromagnetic radiation source. The viewing axis of thetelescope will intersect with the beam of the electromagnetic beam. Inthis arrangement the location of the chemical cloud can be determinedbased on the intersection of the electromagnetic radiation beam and theview axis of the telescope.

[0012] The electromagnetic radiation can be microwave, millimeter wave,infrared, visible, or ultraviolet radiation. The wavelength of theelectromagnetic radiation can be selected to be in resonance with theabsorption lines of the chemicals, or of water vapor or oxygen moleculesthat are commonly present in the cloud. If the wavelength of theelectromagnetic radiation is chosen to be in resonance with theabsorption lines of the target chemical molecules, the returned emissionintensity, as a function of the excitation wavelength, can be used toprovide an increased discrimination of the chemicals against possibleinterference background chemicals.

[0013] Additional objects, features and advantages of the presentinvention will become apparent from the following description andappended claims taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a plan view of a remote spectral analysis sensingsystem, according to a preferred embodiment of the present invention;

[0015] FIGS. 2(a)-2(d) show the spectral footprint of four differentchemical agents; and

[0016]FIG. 3 is a graph with wavelength on the horizontal axis and theabsorption of the agent VX and output intensity of a CO₂ laser on thevertical axis showing the overlapping of a CO₂ laser with absorptionspectrum of the agent VX;

[0017]FIG. 4 is a graph with wavelength on the horizontal axis andabsorption on the vertical axis showing absorption spectrum of sarinrelative to a CO₂ laser tuning range; and

[0018]FIG. 5 is a graph with frequency on the horizontal axis andabsorption coefficient on the vertical axis showing microwave absorptionfrom atmospheric gases.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The following discussion of the preferred embodiments directed toa technique for the remote sensing of a chemical cloud is merelyexemplary in nature, and is in no way intended to limit the invention orits applications or uses.

[0020]FIG. 1 is a plan view of a spectral sensing system 10, accordingto an embodiment of the present invention. The system 10 includes aradiation source 12, a telescope 14 and a spectrometer 16. The radiationsource 12 can be any suitable laser or microwave source consistent withthe discussion herein. The telescope 14 is a Newtonian type telescope,including a collecting mirror 18 for receiving radiation from a sceneand a turning mirror 20. However, the telescope 14 can be any suitabletelescope for receiving and focusing radiation from a scene consistentwith the discussion herein. The spectrometer 16 can be any type ofspectral detecting device that provides a spectral display over apredetermined spectrum, such as an FTIR spectrometer, an acousto-opticspectrometer, or a grating dispersed spectrometer. The source 12,telescope 14 and spectrometer 16 can be mounted within a suitablehousing, and can be included on a platform capable for scanning over awide area for increased surveillance. Additionally, the system 10 can bemade compact and portable to be readily moved from place to place.

[0021] The radiation source 12 emits a radiation beam 22 towards a pairof turning mirrors 24 and 26 to align the beam 22 to be near co-linearwith radiation received by the telescope 14 from the scene. Theradiation beam 22 can be microwave, millimeter wave, infrared, visibleor ultraviolet radiation, depending on the particular application andsystem being used, as will be discussed below. The radiation beam 22 isdirected towards a suspected chemical cloud 28 to excite moleculeswithin the cloud 28 to cause heat to be generated to increase thecloud's temperature relative to the background, usually sky, mountainsor other terrain. In alternate embodiments, the radiation beam 22 can bedirected towards the cloud 28 in a manner that is not co-linear with theemissions from the cloud 28. In this type of design, the location of thecloud 28 can also be determined as the intersection point between theradiation beam 22 and the field of view of the telescope 14.

[0022] The source 12 can be selected so that the wavelength of theradiation beam 22 is in resonance with a particular target molecule ormolecules existing in the cloud being detected. The wavelength of theradiation beam 22 can also be selected to be in resonance with theabsorption lines of water vapor or oxygen molecules commonly present inair. The resonance causes the target molecules, water vapor or oxygenmolecules to rotate or vibrate which causes their energy to increase.Also, electrical transistions may occur in the molecules. The radiationenergy absorbed by the water vapor, the oxygen molecules or the targetmolecules in the cloud 28 is thermalized due to collision energytransfer causing inter-molecular relaxation. At atmospheric pressure,this thermalization is very rapid. This collisional energyredistribution results in heating the molecules in the cloud 28. Anincrease in the temperature of the cloud 28 will increase the emissionintensity of the molecules in the cloud against the background,resulting in an improved detection of the molecules. If the wavelengthof the electromagnetic radiation is chosen to be in resonance with theabsorption lines of the target molecules, the returned emissionintensity, as a function of the excitation wavelength, can be used toprovide an additional way for discrimination against possibleinterference background chemicals. This is because the returned emissionintensity from the target molecules should increase substantially as theexcitation is tuned to the resonance absorption lines of the targetmolecules. In the contrast, the emission intensity from backgroundchemicals should not increase appreciably at these excitationwavelengths.

[0023] The telescope 14 collects thermal emission returned from thecloud 28. The spectral content of the emission is then analyzed by thespectrometer 16. The emission spectrum, typical in the 8-14 micronregion, is used to identify the molecules in the cloud 28 by comparingthe detected emissions to the known “fingerprint” vibrational spectrumof predetermined molecules. Alternatively, an imaging spectrometer, suchas a hyperspectrometer imager, can be used to obtain spatially resolvedspectrum. The contrast from the spatially resolved spectrum can furtherbe used to discriminate against any other interference backgroundchemicals that may be present.

[0024] Several electromagnetic radiation sources, including infraredlasers, such as a CO₂ laser and a DF laser, and microwaves may be usedas the radiation source. The CO₂ laser is a preferred excitation sourcefor detection of chemical agent cloud. This is because severalphosphonate-type chemical agents, including sarin (GB), soman (GD),tabun (GA) and VX, as shown in FIG. 2, have absorption bands in the 9-10μ region that can be excited by a CO₂ laser.

[0025]FIG. 3 illustrates some overlapping of CO₂ laser lines with theabsorption spectrum of agent VX. The upper curve shows the absorptionspectrum of agent VX and the dots on the curve show the overlapping ofthe absorption curve with relatively strong ¹²CO₂ laser lines. The lowercurve shows the intensity distribution of a typical tuned ¹²CO₂ laser.The strong peaks of the VX vapor can be excited by the p-branch of the9.6-μm band or 10.6-μm band of a ¹²CO₂ laser. A single strong CO₂ laserline that overlaps with the strong absorption band may be employed forinitial searching of the target chemicals. Further identification oftarget molecules can be achieved by comparing the returned emission atdifferent CO₂ laser lines against the “fingerprint” absorption spectrumof the target chemicals.

[0026]FIG. 4 shows another example of the overlapping of sarin (agentGB) with CO₂ laser, including both isotopes of ¹²CO₂ and ¹³CO₂. Thisillustrates that strong absorption bands of the chemicals can be excitedby a different isotope of a CO₂ laser.

[0027] Table 1 shows an estimate of the power required for heating a GBvapor cloud by 10° C. This assumes that a CO₂ laser is tuned near thepeak absorption of the vapor, the concentration of the GB is assumed tobe about 100 ppm in the air, the laser beam on the target cloud is about5-cm diameter, and the duration of irradiation is about 1 second. Table1 shows the required CO₂ laser power is relatively low, about 28 W,mainly because of relatively strong absorption cross-sections of thephosphonate bands in the 9-10 μ region. In addition, the range from thesource to the target cloud can be quite long, exceeding more than 5 km,mainly because the air is practically transparent to the CO₂ laser inthe range of concern.

[0028] The DF laser, as well some HF lasers, may also be used as anexcitation source. As shown in FIG. 2, many of the chemical agents haveabsorption bands near the region that overlap with either the DF or HFlaser lines. Table 1 shows that the required DF laser for heating a GBvapor cloud of 100 ppm in the air by 10° C. is relatively mild, about137 W for a duration of 1 second. The estimate again assumes the laserbeam on the target cloud is about 5-cm diameter.

[0029] When CO₂ lasers are used as an excitation source, strongscattered intensity from the CO₂ laser may interfere with the FTIRspectral measurements. It is desirable to turn on the laser for acertain duration and then turn off the laser momentary during the FTIRspectral measurements. Although the CO₂ laser is turned off momentary,the chemical cloud won't cool off immediately, as it takes some time tolose its heat to the surrounding air.

[0030] Microwave radiation may also be used to heat a chemical cloud.One convenient way is to heat the cloud via the excitation of the O₂ orH₂O vapor that is present in the cloud. The energy absorbed by O₂ or H₂Ocan be thermalized rapidly and heat the cloud. Hence, in this approach,a chemical cloud can be heated without prior knowledge of theconstituents and their absorption spectra. FIG. 5 shows the absorptionbands of O₂ or H₂O in air from about 15 GHz to 350 GHz. A FTIRspectrometer can be used to identify the chemical constituents byspectral analyzing the returned radiation in the infrared region fromabout 8 to 14 μm. A microwave source in conjunction with a spectrometeris like providing an “universal detector” capable of detecting variouskinds of chemical vapor. However, a relatively high power microwave maybe needed since a substantial fraction of the power is absorbed by theO₂ or H₂O vapor in transmission through the air before reaching thetarget.

[0031] An alternative way is to excite the chemicals directly by tuningthe microwave frequency in resonance with the chemical absorption lines.The absorption coefficients for chemical warfare agents are generallynot known too well. However, several of other molecules, such as H₂S,SO₂, and CH₃I have been reported. See, for example, N. Gopalsami, S.Bakhtiari, A. C. Raptis, S. L. Dieckman, and DeLucia, “Millimeter-waveMeasurements of Molecular Spectra with Application to EnvironmentalMonitoring,” IEEE Transactions on Instrumentation and Measurement, Vol.45, p. 225-230, 1996.

[0032] Microwave or millimeter wave radiation has a very differentspectral region than infrared radiation, which is the emissions rangeusually analyzed by the spectrometer 16. Therefore, minimal interferencein the measurements from using microwave or millimeter wave radiation isexpected. The spectral range of interest for analyzing the returnedemissions is typically between 8-14 microns, where many chemical vapors,such as certain chemical agents, emit radiation.

[0033] Although the path between the cloud and the sensor unit is alsoheated, if the excitation via water vapor or oxygen absorption lines ischosen, the return emission is expected to have minimal absorption bythe column of the heated air. This is because the absorption by air isquite weak in this spectral region of interest. Although there are weakCO₂ absorption bands in the 8-14 micron region due to the hot-bandabsorption from the μ₂ and 2μ₂ levels of CO₂, these bands are normallyfairly weak near ambient temperature. It is anticipated that theseabsorption bands will not cause any serious interference of the returnedsignal in the spectral range of 8 to 14 μm of interest, if thetemperature rise is kept below 10 to 20° C.

[0034] The required microwave power for the source 12 depends upon thenecessary temperature difference between the cloud 28 and thebackground, the range of the cloud 28, and the absorption coefficientand concentration of the absorber. Table 1 shows an estimate of therequired microwave power and range for heating a 100-ppm GB vapor cloudby 10° C., via absorption of O₂ at about 60 GHz, H₂O at 22.3 GHz, and GBvapor at 90 GHz, for a duration of 1 second. The absorptioncross-section of O2 and H₂O are from Ulaby, et al. The absorption crosssection for GB is taking to be an average value of that of H₂S, SO₂, andCH₃I cited in the above reference by Gopalsami, et al. It is assumedthat the beam 22 can be focused to a spot size of 5-cm diameter at a 200meter range and 10-cm diameter at a 5-km range, and that the source 12is on for 1 second.

[0035] There are trade-offs among choice of different absorbers. Itappears that O₂ yields a relative low required power of about 16 kWmainly caused by relatively high absorption of O₂ in the air. However,the range is likely limited to 200 m. On the other hands, H₂O yields asubstantially high required energy of about 270 kW, although the rangecan exceed 5 km. GB vapor may not be a good choice of absorber becauseof exceedingly high required microwave power. Of course, the requiredmicrowave power can be substantially reduced if the irradiation can beextended over a longer duration TABLE 1 Required Laser of MicrowavePower for Heating 100-ppm GB chemical Cloud by 10° for 1 second durationExcitation source CO₂ laser DF laser Microwave Microwave MicrowaveWavelength ˜9.6 micron ˜3.58 micron ˜60 GHz ˜22.3 GHz ˜90 GHz Absorbingmedium GB vapor cloud GB vapor cloud Oxygen Watervapor(30% H₂O) GB vaporcloud Cross section, cm² 3.4 × 10⁻¹⁸ 0.7 × 10⁻¹⁸ 4.9 × 10⁻²⁴ 1.9 × 10⁻²⁴1.0 × 10⁻²¹ Beam diameter 5 cm 5 cm 5 cm 10 cm 10 cm Range 0-5 Km 0-5 Km0-200 m 0-5 Km 0-5 Km Required Power 28 W 137 W 16 KW 270 KW 380 KW

[0036] One article has investigated the relationship betweenimprovements in detection sensitivity and the temperature differencebetween the target vapor cloud and the background temperature. See T. C.Gruber and J. Ditillo, “Quality assurance measurements for passive FTIRdata collection,” AAPCA Specialty Conference Proceedings SP OpticalRemote Sensing for Environmental and Process Monitoring Proceedings ofthe 1995 Specialty Conference, Sep. 25-27, 1995. The quality in thereturned emission spectrum was expressed as a “score”. For temperaturescloser to or below 2° C., the score is practically zero. Once thetemperature passes a threshold value of ˜2° C., the score increasesmarkedly with increasing temperature difference. Hence, an increase inthe temperature difference by 10° C. or higher, by means of the laser ormicrowave heating, provides an improvement in the sensitivity ofdetecting these molecules substantially.

[0037] The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A system for the detection and analysis of tracechemicals in an aggregate cloud, said system comprising: a radiationsource, said radiation source directing a radiation beam towards thecloud, said radiation beam heating constituents in the cloud; receivingoptics for receiving emissions from the cloud; and a spectrometerresponsive to the emissions from the receiving optics, said spectrometergenerating a spectral display of certain constituents in the cloud.
 2. Asystem for the detection and analysis of trace chemicals in an aggregatecloud, said system comprising: a radiation source, said radiation sourcedirecting a radiation beam towards the cloud, said radiation beamheating constituents in the cloud, said radiation source generating aradiation beam selected from the group consisting of microwave,millimeter wave, infrared, visible, and ultraviolet radiation beams;receiving optics for receiving emissions from the cloud; and aspectrometer responsive to the emissions from the receiving optics, saidspectrometer generating a spectral display of certain constituents inthe cloud.
 3. A system for the detection and analysis of trace chemicalsin an aggregate cloud, said system comprising: a radiation source, saidradiation source directing a radiation beam towards the cloud, saidradiation beam heating constituents in the cloud; receiving optics forreceiving emissions from the cloud; directional optics positioned toreceive the radiation beam from the radiation source and direct theradiation along a direction substantially co-linear with the directionof the emissions received from the cloud; and a spectrometer responsiveto the emissions from the receiving optics, said spectrometer generatinga spectral display of certain constituents in the cloud.
 4. The systemaccording to claim 1 wherein the spectrometer is selected from the groupconsisting of FTIR spectrometers, acousto-optic spectrometers andgrating dispersion spectrometers.
 5. A system for the detection andanalysis of trace chemicals in an aggregate cloud, said systemcomprising: a radiation source, said radiation source directing aradiation beam towards the cloud and generating a radiation beam havinga wavelength that is in resonance with a particular target moleculeexisting in the cloud, said radiation beam heating constituents in thecloud; receiving optics for receiving emissions from the cloud; and aspectrometer responsive to the emissions from the receiving optics, saidspectrometer generating a spectral display of certain constituents inthe cloud.
 6. A system for the detection and analysis of trace chemicalsin an aggregate cloud, said system comprising: a radiation source, saidradiation source directing a radiation beam towards the cloud andgenerating a radiation beam having a wavelength that is in resonancewith absorption lines of at least one of water vapor and oxygenmolecules existing in the cloud, said radiation beam heatingconstituents in the cloud; receiving optics for receiving emissions fromthe cloud; and a spectrometer responsive to the emissions from thereceiving optics, said spectrometer generating a spectral display ofcertain constituents in the cloud.
 7. The system according to claim 1wherein the receiving optics are part of a telescope.
 8. A system fordetecting a chemical agent in a chemical cloud against a sky background,said system comprising: a radiation source, said radiation sourcedirecting a radiation beam towards the cloud to heat constituents in thecloud and raise its temperature relative to the temperature of thebackground, said increase in the temperature of the cloud enhancingpassive emissions from the cloud; a telescope responsive to theemissions from the cloud, said telescope focusing and directing theemissions; and a spectrometer responsive to the emissions from thereceiving optics, said spectrometer generating a spectral display of theconstituents in the cloud.
 9. A system for detecting a chemical agent ina chemical cloud against a sky background, said system comprising: aradiation source generating a radiation beam having a wavelength that isin resonance with a particular target molecule existing in the cloud,said radiation source directing a radiation beam towards the cloud toheat constituents in the cloud and raise its temperature relative to thetemperature of the background, said increase in the temperature of thecloud enhancing passive emissions from the cloud; a telescope responsiveto the emissions from the cloud, said telescope focusing and directingthe emissions; and a spectrometer responsive to the emissions from thereceiving optics, said spectrometer generating a spectral display of theconstituents in the cloud.
 10. The system according to claim 8 whereinthe radiation source generates a radiation beam having a wavelength thatis in resonance with absorption lines of at least one of water vapor andoxygen molecules existing in the cloud.
 11. The system according toclaim 8 wherein the radiation source generates a radiation beam selectedfrom the group consisting of microwave, millimeter wave, infrared,visible, and ultraviolet radiation beams.
 12. A system for detecting achemical agent in a chemical cloud against a sky background, said systemcomprising: a radiation source, said radiation source directing aradiation beam towards the cloud to heat constituents in the cloud andraise its temperature relative to the temperature of the background,said increase in the temperature of the cloud enhancing passiveemissions from the cloud; a telescope responsive to the emissions fromthe cloud, said telescope focusing and directing the emissions;directional optics positioned to receive the radiation beam from theradiation source and direct the radiation along a directionsubstantially co-linear with the direction of the emissions receivedfrom the cloud; and a spectrometer responsive to the emissions from thereceiving optics, said spectrometer generating a spectral display of theconstituents in the cloud.
 13. The system according to claim 8 whereinthe spectrometer is selected from the group consisting of FTIRspectrometers, acousto-optic spectrometers and dispersion spectrometers.14. A method of detecting a chemical agent in a gas cloud against abackground, said method comprising the steps of: heating the gas cloudrelative to the background by directing a beam of radiation towards thecloud to excite constituents in the cloud and increase emissions fromthe cloud; receiving the emissions from the cloud and; and generating aspectral display indicative of the constituents in the cloud.
 15. Amethod of detecting a chemical agent in a gas cloud against abackground, said method comprising the steps of: heating the gas cloudrelative to the background by directing a beam of radiation having awavelength predominantly from the group consisting of microwave,millimeterwave, infrared, visible, and ultraviolet wavelengths towardsthe cloud to excite constituents in the cloud and increase emissionsfrom the cloud; receiving the emissions from the cloud and; andgenerating a spectral display indicative of the constituents in thecloud.
 16. The method according to claim 14 wherein the step of heatingthe cloud includes using a radiation beam having a wavelength that is inresonance with a particular target molecule existing in the cloud. 17.The method according to claim 14 wherein the step of heating the cloudincludes using a radiation beam having a wavelength that is in resonancewith absorption lines of at least one of water vapor and oxygenmolecules existing in the cloud.
 18. The method according to claim 14wherein the step of directing the radiation beam includes directing theradiation beam to be co-linear with the direction of the emissionsreceived from the cloud.